Many novelists have people, places, animals, ect in their head when writing books. Maybe the world is fiction but based on a real place. Or they’ll include an artist who is based on a real artist. It’s very common for authors to have a mental image when writing their books. Not only is it a good go-to reference, but it also helps to imagine scenes, fights, descriptions.
Some notable author casting is E.L. James wanting Robert Pattinson as Christian Grey, according to E! Online. I also remember seeing somewhere that she liked Ryan Gosling, but I can’t remember where I saw that. And J.K. Rowling wanted British actors for Harry Potter. She said it gave the movie some British identity.
I’m no different. When I’m planning out a novel and need a physical embodiment, I look for real people who match my criteria. Then as I write, I try to see how things look. Kind of like a dress-up type thing. How would X actor, who has the right hair, eyes, body type, look in a suit? That’s the magic of Pinterest. I can usually find it.
It’s not easy finding the right looking person. And sometimes there isn’t a person. For my other pen name, I couldn’t find anyone who looked like my hero, Hannibal, so I had an artist over on Deviant Art make one. Again, as I’m writing I can someone to play the role in my head.
Of course these people will most likely never read my books, and there’s a even slimmer chance a movie would be made from my books. But as I write, I am making a movie in my head. Writers have to direct characters on stage, how dialogue is delivered and so forth.